Commercially available PT closures (i.e. those enabling press-on or non-rotary application to a container but requiring rotational or twisting removal) are commonly lined with a conventional plastisol gasket compound that is arranged to be in sealing engagement with the top edge or finish of a container and along the neck portion thereof. When first formed, these containers have a cylindrical bore which is adapted to provide an interference fit with the screw thread on the container neck, thereby enabling the closure to be directly applied to the container without requiring rotation thereof. During thermal processing of the container after the filling or capping thereof, this lining takes a set by which it permanently conforms to the container's helical thread(s) enabling subsequent rotational removal of the closure from the container by the user.
While these commercial closures have found general acceptance in the trade, efforts have been undertaken by those involved in this art to eliminate the need for utilizing a thread-forming lining on the interior of such closures. One such alternative closure construction is shown in U.S. Pat. No. 4,717,034 (Mumford) which describes a one-piece cap shell closure formed of thermoplastic material for capping containers having a plurality of vertically spaced multi-lead threads on the neck surface thereof. The skirt portion of the closure includes a plurality of spaced-apart, flexible and generally vertical thermoplastic ribs which are integral with the skirt each of which is sized to engage a plurality of threads. These ribs are so constructed and arranged that they impart sufficient resistance to cold flow so that the rib only slightly flexes and bends around the threads to form slight indentations on the ribs when such ribs are forced into contact with the threads. These axial-ribbed closures have not, insofar as applicants are aware, found commercial acceptance. The failure of such closures to find such acceptance by the trade is believed to be due to the inability of the vertical ribs to provide sufficient lifting, particularly under vacuum conditions, with conventionally employed thread designs used in currently available glass containers for use with press-on/rotationally removable closures.
The present invention overcomes the problems and disadvantages of these prior art closures and provides a new and improved closure having significant advantages thereover.